<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/paper.js/0.9.24/paper-full.min.js"></script>
    <style>
        #mainCanvas {
            width: 400px;
            height: 400px;
            border: solid 1px black;
        }
    </style>
</head>
<body>
<canvas id="mainCanvas"></canvas>
<script>
    paper.install(window);
    paper.setup(document.getElementById("mainCanvas"));

    let tool = new Tool();
    tool.onMouseDown = function (event) {
        var c = Shape.Circle(event.point.x,event.point.y,20);
        c.fillColor = 'red'
    }
    paper.view.draw();
</script>
</body>
</html>